1. Field of the Invention
The present invention relates to a Josephson-junction logic device and more specifically to a Josephson-junction logic gate based on three or more Superconducting Quantum Interference Devices (SQUIDs) of a magnetically-coupled gate type which ensure electrical isolation of the output from the inputs, wherein the margin of operation is improved
2. Description of the Prior Art
Conventional Josephson logic gates comprising multi-junction SQUIDs can be divided into two types, i.e., a type with a magnetically coupled gate having magnetically coupled control lines as disclosed in U.S. Pat. No. 3,978,351 and a type with a current-injection gate in which the input current is allowed to flow directly into a loop of the SQUID as disclosed in U.S. Pat. No. 4,117,354.
In the magnetically coupled-type gate, an input current is fed to the magnetically coupled control lines i.e., the input signal lines, to change the magnetic field coupled with the loop of the SQUID, whereby the critical current of the gate is changed and a superconductive state is switched to a voltage state. In the conventional magnetically coupled gate, however, the input signal lines are arranged so as to be opposite to all the inductances of the superconducting loop. In this construction, the critical current is simply determined by the sum of the input currents supplied to the input signal lines. That is, the critical current is linearly changed with respect to the sum of the input currents. Because of the linear characteristics, there is a disadvantage in that the allowable ranges of the input currents are small, and, therefore, the margin for operation is small.
The current injection-type gate, in which the input current is allowed to flow directly into the loop of the gate, utilizes a phenomenon in which a superconductive state is switched to a voltage state when the input current exceeds a threshold value. In the current injection-type gate, the critical current is not determined simply by the sum of the input currents. Therefore, the current injection-type gate has nonlinear threshold characteristics, and the boundary between a superconductive state and a voltage state changes more rapidly than that of the magnetically coupled gate, making it possible to improve the operation margin as compared with the operation margin of the magnetically coupled-type gate. This type of gate, however, requires magnetically coupled circuits to electrically isolate the inputs from the output. Because the magnetically coupled circuits are a necessity, there are disadvantages in the current injection-type gate in that the size of the current injection-type gate is large.